The invention relates to a method of opening joints between drilling components in a rock drill which is fed with respect to a feeding beam during drilling and which comprises a frame, a percussion device arranged in the frame. The conventional percussion device comprises a percussion piston which moves in the longitudinal direction, a shank which is arranged in the axial extension of the percussion piston for being struck by the percussion piston and to which the necessary drilling components, such as drill rods, a drill bit and the like are attached to form drilling equipment, and a rotating device which by means of a rotating bushing rotates the shank. The conventional rock drill further comprises at least one axial piston, which is positioned rearwardly of the shank and arranged to move in the axial direction by means of the pressure of a pressure medium fed onto its back surface during drilling, the axial piston acting on the shank by supporting it towards the front part of the drilling machine. The conventional method comprises supporting at least part of the drilling equipment against a supporting surface when threaded joints of the drilling components attached to the shank are opened and striking the drilling equipment with the percussion device, after which the shank is rotated by the rotating device in a direction opposite to normal drilling to open the desired joints.
The invention further relates to a rock drill which comprises a frame, a percussion device provided with a percussion piston which moves in the longitudinal direction, a shank which is arranged in the axial extension of the percussion piston, which is arranged to strike the rear end of the shank, and a rotating device which is arranged to rotate the shank by a rotating bushing arranged around the shank, and drilling equipment consisting of drilling components attached to the front end of the shank. The conventional drilling machine comprises at least one axial piston, which is arranged rearwardly of the shank, there being a pressure channel which extends rearwardly of the axial piston and allows feeding of a pressure medium rearwardly of the axial piston to move the axial piston in the axial direction, the axial piston being arranged to support the shank in the axial direction during drilling.
A drilling machine that comprises a percussion device and a rotating device is used in rock drilling. At the front end of the drilling machine there is a shank which is struck by a percussion piston of the percussion device and rotated by the rotating device during drilling. In the extension of the shank there are drilling components, such as one or more drill rods and a drill bit. The drill bit is the outermost part and comprises buttons for working the rock. The drilling components thus constitute the drilling equipment that is attached to the drilling machine and transmits the percussion force and the rotating force from the shank to the rock to be drilled. The drilling components are attached to the shank and further to other drilling components usually by means of a threaded joint. For example, when the drill bit is changed or drill rods are added/removed between the drill bit and the shank, joints have to be opened. When threaded joints between the drilling components are opened, the shank is rotated by a rotating motor in a direction opposite to normal drilling. To facilitate opening of a joint, the drilling equipment can be struck by the percussion device. In the case of damaged or stuck joints, in particular, it is very common to apply such striking in opening.
Finnish patent no. 98 401 describes a rock drill where the shank can be moved forward by means of axial pistons provided rearwardly of the shank when drilling is adjusted. In that case the shank is supported from behind either directly or by means of a bushing. The pressure acting rearwardly of the axial pistons is changed and thereby the travel of the pistons is adjusted and thus also the position of the shank. There are several axial pistons which are arranged in groups with different travel so that axial pistons with a longer travel can move over the optimal percussion point, to the front of it. It is a known fact that a stroke generated by the percussion piston produces a stress pulse that is reflected back from the rock. In the solution according to the publication, the pulse reflected from the rock is received by axial pistons that extend to the front of the optimal percussion point, and thus the backward movement of the shank and the drilling equipment is dampened. One purpose of this solution is to support the shank and the drilling equipment during the whole duration of drilling by means of axial pistons included in an axial bearing, in which case constant compression stress acts on the drilling equipment, which together with rotation continuously tightens the threaded joints during normal drilling, thus ensuring that the joints between the drilling components remain fastened. This way problems caused by loosened joints can be avoided. The threaded joint is typically damaged by vibration of a loosened joint. In the use of the drilling machine disclosed in the publication it has been noted that it is difficult to open the joints of the drilling components because as the drilling components are struck off, the axial bearing tries to ensure rock contact against the bottom of the drilling hole by supporting the shank during striking, in which case compression stress is ensured in the drilling equipment in the same way as in normal drilling.